This invention relates to filter techniques and, more particularly, to swimming pool filtration devices employing a filter aid to remove suspended solids from water.
Continuous flow filter devices wherein water in the pool is pumped continuously therefrom and into the filter device to clean the water and to return the water to the pool in a continuous cycle are usually sand filters, cartridge filters, diatomaceous earth filters, or a combination of these filters. Sand filters, however, do not remove suspended solids and, in fact, may add to the concentration of suspended solids in the pool. Cartridge filters are expensive, since periodic replacement of the cartridge filter medium is required. By far, the most effective filter for swimming pools is a filter having a filter aid, such as diatomaceous earth, coated on a septum or screen to effectively filter out suspended substances in the water.
Diatomaceous earth filters have a filtration cycle which includes precoating the septum or screen with a relatively thin layer of diatomaceous earth by flowing water through the screen in the filtration direction, filtering impure water by capturing the suspended solids in the water in the minute openings of the diatomaceous earth particles, and, after a predetermined amount of impurities has been collected in the diatomaceous earth, removing the filter cake from the septum. The effectiveness and commercial acceptability of a diatomaceous earth filter depends largely upon the removal of filter cake from the septum and the ease of redepositing the diatomaceous earth onto the septum.
Various techniques have been proposed to accomplish this portion of the filtration cycle. Many techniques which are still employed today involve backwashing, which consists of reversing the flow of water through the filter elements and flushing the spent cake to waste. This technique is expensive since the diatomaceous earth must be added to the filter and since large amounts of water are required to completely clean the septum. Moreover, in communities having a high concentration of swimming pools, diatomaceous earth may tend to clog the sewer systems. Some communities, moreover, have banned the dumping of diatomaceous earth in the community sewer system.
A second type of filter is a regenerative filter which generally does not require backwashing flow but includes elements which can be flexed or mechanically bumped to remove the filter cake, or can be spun to remove the cake by centrifugal force. Filters which are flexed to remove the filter cake are described in U.S. Pat. Nos. 3,100,190 and 3,642,141. Filters which are spun to remove the filter cake are described in U.S. Pat. No. 3,069,014. Spinning a disc-shaped filter element to remove filter cake is not particularly effective, since the spinning action results in little, if any, turbulence of the water within the filter casing, and this action relies primarily on centrifugal force to remove the contaminated diatomaceous earth. The centrifugal force, of course, decreases as the spin axis is approached.